1. Field of the Invention
The present invention relates to engine mounts which include fluid filled elastomeric members and more specifically to such a type of vibration damping mount which includes electrodes which are immersed in an rheopetic fluid and across which a voltage can be applied in order to change the damping characteristics of the device.
2. Description of the Prior Art
FIG. 1 shows a vehicle suspension system in terms of a model having two degrees of freedom. In this model Mp denotes a vibrating body such as an automotive internal combustion engine and transaxle (hereinafter referred to as a power unit), Mb denotes a vehicle chassis on which the engine is supported and G the surface of a road on which the vehicle is running. The engine mounts are depicted as having a spring constant k1 and a damping coefficient c1 while the vehicle suspension is depicted as exhibiting a spring constant k2 and a damping coefficient c2.
This system is such that, if we examine the damping coefficient c parameter in terms of applied vibration frequency and the change in the amount of vibration energy transmitted to the vehicle chassis, it will be apparent from FIG. 11 that about the point P, beyond which the vibration enters a high frequency range, the damping characteristics undergo a change. As the vibration frequency increases into the high frequency range, the value of the damping coefficient increases toward a peak value (note c=infinity trace) and thus attentuation of the vibration level transmitted to the chassis is possible.
It is common to use mounts comprised almost exclusively of elastomeric materials (viz., solid rubber blocks and the like) for mounting an engine and/or transmission arrangement (power unit). However, if it is attempted to increase the damping coefficient c1 of the mounting units, the elastomeric material tends to become harder and the spring constant k1 simultaneously increases. This increases the situation wherein Ftotal (Ftotal=Fc+Fk) cannot be held at desired levels.
In view of this, it has been proposed in JP-A-61-74930 to make use of a liquid column which is defined within an orifice arrangement and which column can be induced to resonate. The orifice in this device is arranged to be variable by utilizing a fluid which exhibits rheopetic properties and by selectively impressing a voltage across electrodes which form a part thereof.
However, with this arrangement even though in a particular frequency range in the low frequency region the dynamic damper functions to control vibration, and in the high frequency region the dynamic spring constant is low, the range in which attenuation of the vibration which is transmitted to the chassis occurs, is narrow. Further, the use of a rheopectic fluid permits the above mentioned range to be enlarged by only a small amount. Hence, no substantial improvement in damping characteristics is achieved by this measure.